Variable-speed generators or, generally speaking, generators having a frequency that diverges from the grid frequency are typically connected to the electrical power grid by means of converters that adapt the voltage and the frequency being generated by the generator to the voltage and the frequency of the electrical power grid. Various devices are used as converters for this purpose such as, for instance, so-called direct converters, with which the two different voltages and frequencies are adjusted relative to each other, for example, using semiconductor switches (e.g. thyristors or gate turn-off thyristors—GTOs) in a direct conversion (AC/AC). Such direct converters exist, for instance, as so-called cycloconverters or as so-called matrix converters (described, for example, in U.S. Pat. No. 5,594,636). In the case of a natural commutation, they generate frequency components of a low-frequency that are undesired and difficult to eliminate, while in the case of forced commutation, they entail large switching losses.
As an alternative, it is possible to ensure a voltage-adapted and frequency-adapted connection of a generator to an electrical power grid in the form of an indirect conversion. With such a conversion, first of all, a rectifier produces a direct current from the alternating current generated by the generator and, in an inverter, this direct current is subsequently matched to the voltage and frequency of the electrical power grid. Such controlled converters likewise make use of semiconductor switches (for instance, GTOs, insulated gate bipolar transistors—IGBTs, metal oxide semiconductor field-effect transistors—MOSFETs, or integrated gate commutated thyristors—IGCTs) and they entail large switching losses at the switching frequencies typically employed.